The present invention relates to apparatus for bending tubes and more particularly concerns an improved die assembly for such apparatus.
In rotary bending apparatus the forward section of a tube is clamped to a rotary bend die by a clamp die and a rearward section of the tube is pressed against the bend die, or some other backup element, by a pressure die. The bend and clamp dies are rotated together with the tube clamped therebetween to wrap the tube around the bend die. In rotary draw bending the tube is stretched beyond its yield point as it is bent. A restraint is placed upon a rear section of the tube, as by the pressure die for example, and at the same time the clamp die presses a forward portion of the tube against the bend die with a force sufficient to insure stretching the outer or convex side of the pipe bend beyond yield.
To achieve adequate pressure on the tube between the clamp and bend die for draw bending, it has been common to press the tube over a significant length of the tube. Commonly the clamp die has a length parallel to the extent of the tube in the order of two to three times the tube diameter. If the clamp die is much shorter than this, the tube is likely to slip relative to the clamp die, or in the alternative, such great force must be exerted by the clamp upon the tube that the tube may be unacceptably deformed. Since it is impossible to make two successive bends in a tube that are significantly closer together than the length of the clamp die, the required use of a long clamp die prevents bending a pipe so as to have short straight pipe sections.
As shown in co-pending applications of Homer L. Eaton for Method and Apparatus for Bending Tube, Ser. No. 614,946, filed Sept. 19, 1975, now abandoned Ser. No. 692,585, filed June 3, 1976, U.S. Pat. No. 4,063,441 and Ser. No. 825,554, filed Aug. 18, 1977, U.S. Pat. No. 4,130,004 all of which are assigned to the assignee of the present invention, a shorter clamp die may be employed in draw bending if a bend is initiated as a compression bend (e.g., without initially stretching the pipe beyond its yield) and after the bend and clamp dies have rotated approximately twenty degrees for example, further restraint is placed on the tube to effect a stretching beyond its yield point. In such an arrangement a short clamp die may be employed to provide sufficient clamping force without unacceptably deforming the tube. Further, in pure compression bending, a shorter length of clamp die may be employed since clamping forces great enough for tube stretching are not required.
Consequently, when using a shorter clamp die, the clamp die may be positioned forwardly of the bend die along the length of the pipe to be bent so that it will exert a force against the pipe directed along a line that is spaced from the axis of rotation of the bend die. This provides a greater moment arm for the clamping force and results in less deformation of the pipe by the clamp die. Where the longer clamp dies have been used on the other hand, it has been the custom to position the clamp die as far rearwardly as possible so that the rear end of the clamp die is substantially at the point of tangency between the pipe and the bend die. This was necessary with a long clamp die so that, where required in a pipe with multiple bends, successive bends could be made as close together as possible. However, even with a long clamp die, improved rotary bending and in particular less deformation of the pipe, will occur and less force need be exerted by the clamp die, if the clamp die can be moved forwardly in the plane of bend to space the point of application of clamp die force from the bend die axis.
Although it is desirable to perform a bending operation with a more forwardly positioned clamp die, it is not possible, as previously mentioned, to employ this clamp die position when a bend is to be made so close to a previously made bend that the straight portion of pipe between the two successive bends is not significantly greater than the length of the clamp die. For this reason in the past, the clamp die has been positioned substantially at the point of tangency of the pipe and the bend die despite the fact that for many bends this is not an optimum position.
Accordingly it is an object of the present invention to avoid or minimize above-mentioned problems and to improve clamp die operation.